Our studies of liver slice to kidney bed grafts in inbred strains of mice have continued to yield useful information pertaining to the influence of various histocompatibility barriers in the course of rejection of this tissue. We have found that, within the major histocompatibility complex two subregions alone are responsible for rapid, acute graft rejection. Antigens controlled by the other subregions of the complex when they differ in the graft donor and recipient, cause a chronic rejection pattern much like that resulting from differences in minor histocompatibility antigens. We now have a detailed histological picture of the fate of a syngeneic chronically rejecting and acutely rejecting graft at twenty-four hour periods during the first fourteen days after grafting. We find distinct, repeatable differences in disappearance of liver tissue, infiltration pattern of immunologically competent cells and scar formation. We have begun to study the donor-recipient origins of the cells in this time sequence of grafts using immunofluorescent techniques and find that there may be some doubts as to the generally accepted tissue localization of H-2 antigenic sites. The techniques developed during the study and the resulting findings may have considerable significance for the understanding and prevention of human graft rejection and the elucidation of major histocompatibility complex function. BIBLIOGRAPHIC REFERENCES: Schultz, Jane S., Beals, Theodore F., and Petraitis, Frances P. 1976. Tissue Graft Rejection in Mice. I. Contributions of H-2 and Non-H-2 Genetic Barriers. Immunogenetics 3, 85. Schultz, Jane S., Beals, Theodore F., and DeMott-Friberg, Roberta. 1977. Tissue Graft Rejection in Mice. II. Graft Survival Across H-2 Regional Barriers. Immunogenetics. (In press)